The photolithography for production of the semiconductor devices and others is implemented using a projection exposure apparatus for projecting a pattern image of a mask (or a reticle) through a projection optical system onto a wafer (or a glass plate or the like) coated with a photoresist or the like. The resolving power (resolution) required for the projection optical system of the projection exposure apparatus is becoming increasingly higher and higher with increase in integration degree of the semiconductor devices and others.
As a result, in order to satisfy the requirements for the resolving power of the projection optical system, it is necessary to shorten the wavelength λ of illumination light (exposure light) and to increase the image-side numerical aperture NA of the projection optical system. Specifically, the resolution of the projection optical system is expressed by k·λNA (where k is the process coefficient). The image-side numerical aperture NA is represented by n·sinθ, where n is a refractive index of a medium (normally, gas such as air) between the projection optical system and the image plane and θ a maximum angle of incidence to the image plane.
In this case, if the maximum incidence angle θ is increased in order to increase the numerical aperture NA, it will result in increasing the input angle to the image plane and the output angle from the projection optical system, so as to increase reflection loss on optical surfaces and thus fail to secure a large effective image-side numerical aperture. For this reason, there is the known technology of increasing the numerical aperture NA by filling a medium like a liquid with a high refractive index in the optical path between the projection optical system and the image plane.
However, application of this technology to the ordinary dioptric projection optical systems caused such disadvantages that it was difficult to well correct for chromatic aberration and to satisfy the Petzval's condition to well correct for curvature of field, and that an increase in the scale of the optical system was inevitable. In addition, there was another disadvantage that it was difficult to secure a large effective image-side numerical aperture while well suppressing the reflection loss on optical surfaces.